(1) Field of the Invention
The present invention relates to a translation movement guidance mechanism with positional locking especially for adjustable elements of an automobile vehicle seat.
(2) Prior Art
A typical example of the application of such a mechanism is the installation of a height adjustable headrest for such a seat, but such a mechanism can also be used, for example, for adjusting the length of the seat cushion, adjusting the length of an armrest, etc.
For the height adjustment of a headrest, a system of stems which slide in guide bushes is commonly used. The stems can be attached to the headrest and slide in fixed bushes at the top of the backrest or the stems can be attached to the backrest or at least securely held at the top of the backrest and then the headrest includes bushes which slide over the stems.
Such a system also includes relative positional locking means to, on the one hand, ensure that the headrest is held in the position selected by the user, to improve user's comfort, and, on the other hand, to ensure that the neck of the passenger is supported if the vehicle is subjected to a shock.
For example, notch adjustment systems are known in which a lock in fixed position in relation to the bushes engages with notches made in the stems. The lock and notch system can be arranged in the form of a ratchet lock to enable easy adjustment by simply pulling the headrest upwards but allowing downwards movement of the headrest only by a voluntary unlocking operation to prevent the headrest from moving downwards in the case of an accident. A notch system does not allow continuous positional adjustment and, in addition, requires the making of notches on the stems which increases the cost.
Other systems known, for example by FR-2803563, US-2002/0038969, FR-2833052, allow continuous adjustment, the locking being generally obtained by jamming or buttressing: a thin blade, hinged along an axis offset and orthogonal in relation to that of the stem, or a flexible blade, includes a hole through which the stem passes with sufficient clearance to allow it to slide when the blade is placed in a position more or less perpendicular to the stem; however, when the blade, which comprises the locking component, is held inclined, it blocks the stem in its position, the diametrically opposite edges of the hole forming more or less parallel bearing elements which exert a gripping force on two diametrically opposite sides of the stem. The locking force is increased when a higher sliding force is applied, due to the resulting buttressing effect of the blade.
These systems have however the disadvantage of causing aggressive engagement of the edge of the locking blade hole with the stem, which over time damages the surface of the stem.
Another problem comes from the fact that the stems are not always straight and, in particular, they can sometimes be significantly elbowed for reasons of seat installation and or required headrest sliding direction.
It then becomes difficult, if not impossible, to pass the stems through the guide bushes when installing the headrest. For example, a headrest may be installed on a U-folded tube, on U bottom side, which is therefore placed upside down. The arms of the U, facing downwards, comprise the two stems the lower ends of which are attached to the upper part of the backrest. The height adjustment of the headrest is obtained by the sliding of the headrest over the said stems which are fixed in relation to the backrest. In such a case, it becomes impossible to install the headrest on the stems by sliding it from their free ends when the stems are elbowed for example near their centres.
Similar problems can occur in sliding seat or armrest element assemblies as stated previously.